The regulated assembly and disassembly of microtubule structures is a major aspect of the eucaryotic cell cycle. This dynamic behavior is thought to be regulated in vivo by the action of microtubule-associated proteins (MAPs). Although MAPs have been studied extensively in vitro little is known about their in vivo functions. To be able to understand the role of MAPs in vivo I am studying BIK1, the only characterized non-motor MAP from the yeast, S cerevisiae. The BIK1 protein associates with microtubules in vivo, and this association appears to be restricted to a specific interval in the cell cycle (G2/M). bikl null mutants have phenotypes suggesting that BIK1 plays a role in several aspects of mitotic microtubule function. I am taking two approaches to studying BIK1 function in vivo. The first approach is an analysis of the regions of BIK1 required for association with microtubules and the regions required for the cell cycle dependent change in localization. This will be done by examining the localization of in vitro constructed BIK1 mutants. I will also look for cell cycle dependent modifications of BIK1. The second approach is the analysis of mutants that I have isolated (slb mutants) that require the expression of BIKI for viability, and therefore encode overlapping functions with BIK1. Two of these genes will be cloned, SLB1 and SLB2. I describe experiments to understand the function of BIK1 through an analysis of the physiologic defect of the bik1 slb double mutants. The final goal of this project is to identify BIK1-like genes from human cells and from pathogenic fungi. This will be done by hybridization, or by functional complementation of bik1 slb double mutants. The isolation of such genes would have implications for the development of antineoplastic and antifungal drugs.